There is a temperature limiter inside the handle. At one time your cold water was warmer, (like in the summer time), so the ratio of hot to cold at the setting you have gave hot water. Now the water, in the winter time, is colder so that same amount of hot water is chilled to a lower temperature and you get warm water. Take the handle button off and reset the limiter cam to a hotter position.

: Recently, the maximum hot water temp that I can get from my shower seems less than it once was. I have the valve turned all the way counter clockwise, and the water seems tepid. I've tried adjusting the valve so that it will turn more to the left, but it just cycles back to the off position with no more heat available. Does the Moen Positemp valve have some sort of antiscalding safety thing built into it which prevents too hot of water? Can this wear out over time and lower the maximum attainable temperature? Is there some other way to adjust this thing, or do I have to have a plumber come out and weld in a new valve? Thanx for any advice you can offer...Brett